ff_flac_parse_picture() parses a buffer containing a flac metadata
picture block by wrapping it in an AVIOContext and using the AVIOContext
API. Consequently, when not enough data could be read AVERROR(EIO) was
returned although reading didn't really fail: A block that contains a
subfield whose size field indicates that it is so big as to extend
beyond the buffer is just invalid.

This commit changes this by using the bytestream2 API instead;
furthermore, the checks for whether there is enough data left are
performed before allocating a buffer for said data.

Finally, if the length of the picture description is bigger than
INT_MAX, it will now raise an error.

During parsing a flac picture metadata block, the mimetype is read as
follows: Its 32b size field is read and checked for being in the range
1..63; afterwards, the actual mimetype-string is read into a buffer of
size 64, where the length to read is the minimum of the length field and
the size of the destination buffer -1. Then an assert guards that length
is indeed < the size of the destination buffer before the string in the
buffer is zero-terminated.

The FFMIN as well as the assert are actually redundant, as it has
been checked that the string (even after terminating) fits into the
buffer. In order to make this clear, reword the check "len >= 64" to
"len >= sizeof(mimetype)" and drop the FFMIN as well as the assert.

Put an AVIOContext whose lifetime doesn't extend beyond the function where
it is allocated on the stack instead of allocating and freeing it. This
also avoids the need to free it, which in this case fixes possible
memleaks on error.

Up until now, using a dynamic buffer entailed at least three
allocations: One for the AVIOContext, one for the AVIOContext's opaque
(which, among other things, contains the small write buffer), and one
for the big buffer that is independently allocated that is returned when
calling avio_close_dyn_buf().

It is possible to avoid the third allocation if one doesn't use a
packetized dynamic buffer, if all the data written so far fit into the
write buffer and if one does not require the actual (big) buffer to have
an indefinite lifetime. This is done by making avio_get_dyn_buf() return
a pointer to the data in the write buffer if nothing has been written to
the main buffer yet. The dynamic buffer will then be freed using
ffio_free_dynamic_buffer (which needed to be modified not to call
avio_close_dyn_buf() internally).

The G729 reference decoder clips after each individual operation and keeps track if overflow
occurred (in the fixed point implementation), this here is
simpler and faster but not 1:1 the same what the reference does.

Non fuzzed samples which trigger any such overflow are welcome, so
the need and impact of different clipping solutions can be evaluated.

c0c79461967 unintentianally changed the initialization flow of the
decoder: It caused the capture buffers to be initialized on
v4l2_m2m.c:180 in v4l2_configure_contexts(). This breaks h264 decoding
on the Odroid XU4 (RPI4 was not affected).

This commit postpones capture buffer initialization
as before c0c79461967 to fix the issue.